1. Field of the Invention
The present invention relates to a printing apparatus for receiving document data in which binary data and multi-value data mixedly exist from an external apparatus such as a host computer or the like and for printing.
2. Related Background Art
Hitherto, in document data in which characters and images mixedly exist, generally, the character data is constructed by binary data and the image data is constructed by multi-value data. In the case of printing such document data using a printer, multi-value data such as an image or the like is first converted into a binary pattern using a dither method or the like and is developed into a bit map memory. In the bit map memory, the binary pattern is synthesized with the binary data (characters or the like) and the resultant data is generated for printing.
However, in the case where multi-value data is converted into binary data, is developed into the binary bit map memory, and is then printed, so long as a process to convert it from multi-value data into binary data is executed by software, the burden on the CPU increases significantly, processing time increases, and throughput of the printer deteriorates. If hardware to convert from the multi-value data into the binary data is provided, circuit scale increases and the costs rise.
If data of the full size as much as up to one page is handled as multi-value data, a bit map memory to develop the multi-value data of one page as a binary pattern is needed. Assuming that a capacity of the bit map memory is set to, e.g., 150 d.p.i. and multi-value data of 64 gradations is substituted for by the binary data by a density pattern method or the like, a printer which can print at a resolution of 1200 (=150.times.8) d.p.i. (namely, 64 gradations for 8.times.8 dots) is necessary. If image data for an area of A4 size is stored at a resolution of 1200 d.p.i., a memory capacity of 16 Mbytes is needed, so that such a high resolution is unpractical in terms of cost.
On the other hand, if independent memories for the multi-value data and the binary data are used and the multi-value data has a resolution of 150 d.p.i. and 64 gradations (6 bits/pixel), a page memory to store the multi-value data of a capacity of about 1.6 Mbytes (.apprxeq.256 kbytes.times.6) corresponding to one page of A4 size is needed. As shown in FIG. 4, however, since the multi-value image data generally exists only in a part of a given page, in most cases, most of the areas in the page memory of 1.6 Mbytes are not used, and thus this approach is uneconomical.